X-ray tube having anode target layer of molybdenum rhenium alloy

ABSTRACT

An x-ray tube is described which includes a rotary anode having a target layer of molybdenum rhenium alloy provided on a base member of molybdenum. As a result of the improved target layer, the x-ray tube has a greatly increased useful life due to a lower reduction in x-ray radiation after multiple exposures at high thermal loading of the target.

United States Patent [191 Gager X-RAY TUBE HAVING ANODE TARGET LAYER OF MOLYBDENUM RHENIUM ALLOY Inventor: Robert M. Gager, Elmhurst, Ill.

Assignee: Picker Ohio Filed: May 18, 1972 Appl. No.: 254,502

Corporation, Cleveland,

U.S. Cl.....-..; ..3l3/60, 313/330 Int. Cl ..H0lj 35/10 Field of Search ..313/60, 330

References Cited UNITED STATES PATENTS 8/1968 Elsas ..3l3/60 MOLYBDENUM 1 June 5,3973

Primary ExaminerRoy Lake Assistant ExaminerDarwin R. Hostetter Attorney-Stephen W. Blore, Kenneth S. Klarquist, Joseph B. Sparkman et al.

[57] ABSTRACT An x-ray tube is described which includes a rotary anode having a target layer of molybdenum rhenium alloy provided on a base member of molybdenum. As a result of the improved target layer, the x-ray tube has a greatly increased useful life due to a lower reduction in x-ray radiation after multiple exposures at high thermal loading of the target.

10 Claims, 3 Drawing Figures RHENIUM- MOLYBDENUM ALLOY PATENTED JUN 1 75 FIG. 2

FIG. 5

RHENRUM MOLYBDENUM ALLOY MOLYBQENUM X-RAY TUBE HAVING ANODE TARGET LAYER OF MOLYBDENUM RI-IENIUM ALLOY BACKGROUND OF THE INVENTION The subject matter of the presentinvention relates generally to x-ray tubes and in particular to x-ray tubes employing rotary anodes for generating x-rays having wave length characteristics desirable in the examination of soft tissue, such as in womens breasts. Molybdenum is currently favored for such anodes but tends to crack and roughen which results in reduction of the x-ray radiation. I have found that an alloy of molybdenum and rhenium reduces this problem substantially.

Previous x-ray tubes, such as that of U.S. Pat. No. 3,328,626 of Natter et al., granted June 27, 1967, have employed rotating anodes including base members of molybdenum which may also contain rhenium. However, the x-ray emitting target portion of such anode is made of tungsten which is subject to x-ray radiation dropoff due to cracking and roughening of the target. Other prior x-ray tubes such as that of U.S. Pat. No. 2,863,083 of Schram have employed rhenium in a thin layer over the target and the rest of the anode in order to increase the cooling speed of such anode because of the greater thermal emissivity of rhenium. However, this is not satisfactory because the rhenium layer shields the target layer from most of the electrons and emits x-rays of a different wavelength than the target layer which results in poor contrast of the x-ray images recorded on film or viewed on a fluorescent screen.

In order to overcome the above disadvantages and to provide an anode for an x-ray tube having a long useful lifetime with small reduction in x-ray radiation emission at high thermal loading, it has been found that the anode target should be made of a molybdenum rhenium alloy containing approximately 2 to 15 percent by weight rhenium and the balance molybdenum. This target may be a layer of molybdenum rhenium alloy on an anode base member of molybdenum, such target layer having a thickness on the order of about 0.04 inch. An x-ray tube containing a rotary anode employing such a target layer of percent rhenium and 90 percent molybdenum was tested using 4,000 exposures each of 4 seconds duration at 60 kilovolts and 250 milliamperes. X-ray radiation density measurements showed a radiation dropoff of only 10 to percent for this tube compared with a radiation dropoff several times greater than this in the case of pure molybdenum targets under similar conditions.

It is, therefore, one object of the present invention to provide an improved x-ray tube of longer useful lifetime.

Another object of the invention is to provide such an x-ray tube having an anode which may be subjected to high thermal loading by electron bombardment during repeated exposures without greatly reducing the x-ray radiation level.

A further object of the present invention is to provide such an x-ray tube in which at least the target portion of its anode is made of a molybdenum rhenium alloy.

Still another object of the invention is to provide such an x-ray tube in which the target portion is a layer on an anode base member of molybdenum.

An additional object of the invention is to provide such an x-ray tube having an x-ray wavelength spectrum which is substantially the same as that of pure molybdenum.

BRIEF DESCRIPTION OF DRAWINGS Other objects and advantages of the present invention will be apparent from the following detailed description of a preferred embodiment thereof and from the attached drawings of which:

FIG. 1 is a plan view of an x-ray tube made in accordance with the present invention with a portion of the envelope broken away for purposes of clarity;

FIG. 2 is a section view taken along the line 2-2 of FIG. 1 showing the top side of the anode of the x-ray tube; and

FIG. 3 is a vertical section view taken along the line 33 of FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENT As shown in FIG. 1, an x-ray tube in accordance with the present invention includes a cathode 10 and an anode 12 mounted within an evacuated glass envelope 14. The anode may be a rotary anode which is mounted for rotation within a suitable bearing sleeve 16 in a conventional manner. The cathode 10 is a thermionic cathode such as a heated filament of tungsten or thoriated tungsten which is mounted within a cathode cup type of focusing electrode 18. Thus, electrons emitted from cathode l0 bombard a target portion 20 of the rotating anode 12. The target portion 20 may be a frustroconical track which is rotated in the direction of arrow 21 past the electron beam. The electrons are focused to strike the target portion 20 at a focal spot 22 from which x-rays are emitted through the glass envelope 14.

As shown in FIGS. 2 and 3, the rotating anode 12 includes an anode body member 24 of a refractory metal, such as molybdenum, and a target layer 26 of molybdenum rhenium alloy coated on at least the target portion 20 of the anode. The x-ray anode 12 is fixedly mounted on a support rod 28 in any suitable manner. It has been found that the molybdenum rhenium alloy should contain between about 2% and 15% by weight rhenium, with the balance molybdenum. The target layer should be relatively thick and may have a thickness between about .03 and .08 inch. The molybdenum rhenium target layer greatly increasesthe useful life of the x-ray tube by reducing the radiation dropoff that occurs after numerous exposures at high thermal loading. This improvement is apparently due to the fact that the addition of rhenium reduces recrystalization and grain growth which tends to cause surface cracking. Such cracking and other surface damage reduces x-ray radiation due to self absorption of the x-rays within the target.

The anode may be manufactured by compacting molybdenum powder in a suitable dye cavity, followed by sintering and forging of the sintered metal body to give it the desired final shape and high density in a similar manner to that shown in U.S. Pat. No. 3,136,907 of Kieffer et al granted June 9, 1962. The target layer 26 is formed by a mixture of molybdenum powder and rhenium powder, or rhenium coated molybdenum particles, provided as a layer within the dye cavity prior to compacting and sintering. Of course it should be understood that the entire anode may be made of rhenium molybdenum alloy but this is not very practical for economic reasons.

In one embodiment of the invention, an x-ray tube having a rotary anode base of molybdenum and a target portion of an alloy of 10 percent rhenium and percent molybdenum was tested with 4,000 exposures at 60 kilovolts and 250 milliamperes while rotating at 60 revolutions per second, each exposure cycle having an exposure time of 4 seconds and a cooling time of 86 seconds between exposures. The x-ray radiation density at the end of 4,000 exposures showed a reduction of only to percent from the initial radiation level compared with a radiation dropoff several times greater than this for pure molybdenum targets under similar conditions.

It will be obvious to those having ordinary skill in the art that many changes may be made in the abovedescribed preferred embodiment of the invention without departing from the spirit of the invention. For example, for some purposes a fixed anode rather than a rotating anode can be employed and the anode base member may be a refractory metal other than molybdenum. Therefore, the scope of the invention should only be determined by the following claims.

I claim:

1. An x-ray tube in which the improvement comprises:

an evacuated envelope;

a cathode mounted within said envelope; and

an anode mounted'within said envelope and having a target portion which emits x-rays when bombarded by electrons from said cathode, said target portion consisting of a molybdenum rhenium alloy.

2. An x-ray tube in accordance with claim 1 in which said alloy contains between 2 and 15 percent rhenium.

3. An x-ray tube in accordance with claim 1 in which said target portion is a target layer of molybdenum rhenium alloy on an anode base member of a different metal than said layer.

4. An x-ray tube in accordance with claim 3 in which the anode base member is made of molybdenum.

5. An x-ray tube in accordance with claim 1 in which the anode is mounted for rotation.

6. An x-ray tube in accordance with claim 4 in which said target layer has a thickness greater than about 0.03 inch.

7. An x-ray anode in which the improvement comprises:

an anode member having a target portion which emits x-rays when bombarded by electrons, said target portion being made of a molybdenum rhenium alloy.

8. An anode in accordance with claim 7 in which said target portion is a target layer of molybdenum rhenium alloy on an anode base member of a different metal than said layer.

9. An anode in accordance with claim 8 in which the base member is made of molybdenum.

10. An anode in accordance with claim 7 in which said alloy contains between 2'and 15 percent rhenium. k 

2. An x-ray tube in accordance with claim 1 in which said alloy contains between 2 and 15 percent rhenium.
 3. An x-ray tube in accordance with claim 1 in which said target portion is a target layer of molybdenum rhenium alloy on an anode base member of a different metal than said layer.
 4. An x-ray tube in accordance with claim 3 in which the anode base member is made of molybdenum.
 5. An x-ray tube in accordance with claim 1 in which the anode is mounted for rotation.
 6. An x-ray tube in accordance with claim 4 in which said target layer has a thickness greater than about 0.03 inch.
 7. An x-ray anode in which the improvement comprises: an anode member having a target portion which emits x-rays when bombarded by electrons, said target portion being made of a molybdenum rhenium alloy.
 8. An anode in accordance with claim 7 in which said target portion is a target layer of molybdenum rhenium alloy on an anode base member of a different metal than said layer.
 9. An anode in accordance with claim 8 in which the base member is made of molybdenum.
 10. An anode in accordance with claim 7 in which said alloy contains between 2 and 15 percent rhenium. 